Child & Teen BMI Calculator

About Child & Teen BMI Calculator

How Child BMI Differs From Adult BMI

Children's BMI is interpreted as a percentile rather than a fixed number because body composition changes throughout growth. A BMI of 18 is normal for a 13-year-old girl but high for a 5-year-old girl. The CDC and the American Academy of Pediatrics (AAP) recommend comparing each child's BMI to a reference population of children the same age and sex, then expressing the result as a percentile. The same formula applies (weight in kg divided by height in metres squared), but the interpretation is age- and sex-specific.

The CDC published five percentile cut-offs that define weight categories for children aged 2 to 19:

Percentile	Category	Clinical meaning
Below 5th	Underweight	May warrant a check on growth, nutrition, or underlying conditions
5th to 84th	Healthy weight	BMI is in the typical range for age and sex
85th to 94th	Overweight	Excess weight relative to peers; lifestyle review recommended
95th to 119% of 95th	Obesity	Clinical assessment recommended per AAP guidelines
120% of 95th or higher	Severe obesity	CDC extension added in 2022 for very high BMI

The severe obesity tier was added because the original 95th percentile curve flattens at very high BMI, meaning a child at the 99th percentile and a child far above it both register as "the 99th". Expressing severe cases as a percentage of the 95th percentile gives a sharper picture, as described in the 2023 AAP Clinical Practice Guideline on the evaluation and treatment of children with obesity.

How the LMS Method Calculates Percentiles

The CDC growth charts use the LMS method, which models the BMI distribution at each age using three parameters: L (a power that adjusts for skewness), M (the median BMI), and S (the coefficient of variation). A child's Z-score is then converted to a percentile using the standard normal distribution.

The formula when L is not zero:

Z = ((BMI / M)^L - 1) / (L × S)

And when L is approximately zero:

Z = ln(BMI / M) / S

Worked example 1 (boy). An 8-year-old boy who is 130 cm tall and weighs 30 kg has a BMI of 30 / 1.30² = 17.75 kg/m². Looking up the CDC LMS values for boys at 96 months (L = -3.183, M = 15.78, S = 0.1021), the calculation gives Z ≈ 0.96, which corresponds to roughly the 83rd percentile. That sits just below the 85th overweight threshold, so the category is "healthy weight" - but close to the boundary, so worth tracking over time.

Worked example 2 (girl). A 12-year-old girl who is 152 cm tall and weighs 45 kg has a BMI of 45 / 1.52² = 19.48 kg/m². The CDC LMS values for girls at 144 months (L = -1.976, M = 18.10, S = 0.148) give Z ≈ 0.46, which is about the 68th percentile. That is comfortably inside the healthy range (5th to 85th). This calculator does the LMS lookup, Z calculation, and percentile conversion automatically using these same parameters.

What Are Current Childhood Obesity Rates?

About 21.1% of US children and adolescents aged 2 to 19 had obesity in the August 2021 to August 2023 NHANES survey, with another 15.1% classified as overweight and 7.0% with severe obesity, according to CDC NCHS Health E-Stat 112 (February 2026 release). That is up from 13.9% obesity in 1999 to 2000. Prevalence varies sharply by age, ethnicity, and family income:

Group	US obesity prevalence	Source / period
Ages 2 to 5	14.9%	NHANES Aug 2021-Aug 2023 (Table 2)
Ages 6 to 11	22.1%	NHANES Aug 2021-Aug 2023 (Table 2)
Ages 12 to 19	22.9%	NHANES Aug 2021-Aug 2023 (Table 2)
Hispanic children	26.2%	CDC Childhood Obesity Facts (2017-March 2020 cycle)
Non-Hispanic Black children	24.8%	CDC Childhood Obesity Facts (2017-March 2020 cycle)
Non-Hispanic White children	16.6%	CDC Childhood Obesity Facts (2017-March 2020 cycle)
Non-Hispanic Asian children	9.0%	CDC Childhood Obesity Facts (2017-March 2020 cycle)
Family income below 130% of FPL	25.8%	CDC Childhood Obesity Facts (2017-March 2020 cycle)
Family income above 350% of FPL	11.5%	CDC Childhood Obesity Facts (2017-March 2020 cycle)

In England, NHS Digital's National Child Measurement Programme reported that 9.6% of children in reception (aged 4 to 5) were obese in the 2023/24 school year, rising to 22.1% in Year 6 (aged 10 to 11). Obesity in reception was more than twice as high in the most deprived areas (12.9%) compared with the least deprived (6.0%). For Year 6, the gap was 29.2% versus 13.0%. The doubling of obesity prevalence between reception and Year 6 is one of the starkest patterns in UK health data.

How Should Parents Interpret a Child's BMI Percentile?

A single percentile reading is a screening signal, not a diagnosis. The AAP's 2023 Clinical Practice Guideline emphasises that paediatricians should look at growth trajectory over multiple visits, family history, comorbidities such as blood pressure and lipids, and the child's lifestyle and environment - not just the BMI number on a single day. Some practical guidance:

• Track trends, not single readings. A child whose BMI percentile is stable around the 75th is in a different situation from one jumping from the 50th to the 90th in twelve months.
• Growth spurts cause noise. Children sometimes gain weight before a height growth spurt. A short-term jump in BMI percentile may smooth out within a year.
• Athletes are misclassified. Like adults, muscular adolescents can register as overweight on BMI despite low body fat. The Body Fat Calculator can offer an additional perspective on body composition.
• Family history matters. Children of parents with obesity are at higher metabolic risk at lower BMI percentiles. AAP guidelines suggest tighter monitoring in this group.
• Don't put kids on adult diets. Restrictive calorie targets aimed at adults can interfere with growth and bone development. Paediatricians prefer family-based lifestyle interventions.

Quick-Reference BMI Cut-Offs by Age and Sex

The cut-offs below give the BMI value at each key percentile for boys and girls at common ages, calculated from the CDC LMS parameters. Use this as a sanity-check or a reference for tracking your child's growth between visits. The middle column is the median BMI for that age and sex.

Age	Sex	5th (underweight cut)	50th (median)	85th (overweight cut)	95th (obesity cut)
3	Boy	14.3	16.0	17.3	18.2
3	Girl	14.0	15.7	17.2	18.3
5	Boy	13.8	15.4	16.8	17.9
5	Girl	13.5	15.2	16.8	18.3
8	Boy	13.8	15.8	18.0	20.1
8	Girl	13.5	15.8	18.3	20.7
10	Boy	14.2	16.6	19.4	22.2
10	Girl	14.0	16.9	20.0	23.0
12	Boy	15.0	17.8	21.0	24.2
12	Girl	14.8	18.1	21.7	25.3
14	Boy	16.0	19.2	22.7	26.0
14	Girl	15.8	19.4	23.3	27.3
16	Boy	17.1	20.6	24.2	27.6
16	Girl	16.8	20.5	24.7	28.9
18	Boy	18.2	21.9	25.7	29.0
18	Girl	17.6	21.3	25.7	30.3

Note that the underweight cut at age 18 (BMI 18.2 for boys, 17.6 for girls) is still well below the adult underweight threshold of 18.5. This is one reason older teens are sometimes flagged as "underweight" by adult BMI tools when the paediatric chart says they are at the 5th to 10th percentile - still low, but normal for their age. By age 20, the standard adult BMI Calculator is the right tool.

What Is the Difference Between CDC and WHO Charts?

The CDC charts used in this calculator are the US standard for ages 2 to 19 and were originally published in 2000. The WHO publishes a separate set of standards for children aged 0 to 5 (the WHO Child Growth Standards) and a reference for ages 5 to 19. The two systems can give different percentile results for the same child, especially under age 5:

Chart	Age range	Population basis	Recommended for
CDC 2000 Growth Charts	2 to 19	US NHANES surveys 1963-1994	US, ages 2 and over (standard since 2010)
WHO Child Growth Standards	0 to 5	Six countries, breastfed children	US and most of world, under age 2 (CDC recommends WHO under 2)
WHO Growth Reference	5 to 19	Reconstructed from 1977 NCHS data	International use, especially outside the US
UK-WHO Growth Charts	0 to 4 then UK 1990	WHO 0-4, UK 1990 reference 4+	UK (Royal College of Paediatrics and Child Health)

In the US, CDC officially recommends using WHO charts under age 2 and CDC charts from age 2 onward. The UK uses a hybrid (UK-WHO) that switches to a UK-specific reference at age 4. Because this tool covers ages 2 to 19, the CDC chart is the appropriate reference for both US and most international users at these ages. The percentile produced for a UK child will be very close to the UK-WHO chart result for the same inputs (typically within 1 to 2 percentile points).

Common Mistakes When Interpreting Child BMI

• Comparing to adult thresholds. A BMI of 22 in a 7-year-old places them above the 95th percentile (obesity range), even though 22 sounds "normal" by adult standards.
• Ignoring the months. For a 4-year-old, the difference between 48 months and 59 months can shift the percentile by several points. Use exact age including months for accuracy.
• Reading one number in isolation. The CDC growth chart is meant to be read longitudinally. A single point matters far less than the slope between points.
• Mixing CDC and WHO charts. If your paediatrician uses WHO under age 2 and CDC from age 2, percentile values will shift slightly at the transition. That is the chart change, not the child changing.

For older teenagers transitioning to adult care, the standard BMI Calculator with fixed adult thresholds becomes the right tool from around age 18 to 20. For families wanting to understand a child's typical calorie needs (which depend on age, weight, height, and activity), the Calorie Calculator provides estimates - though paediatric energy needs are highly variable and a registered dietitian is the right source for any meaningful restriction or goal-setting.

Privacy Note

Height, weight, age, and sex are processed entirely in your browser. Nothing is sent to a server. The LMS reference data is embedded in the page itself.

Sources

• CDC - Percentile Data Files and LMS Parameters (bmiagerev.xls)
• CDC - Clinical Growth Charts (Ages 2-20)
• CDC NCHS Health E-Stat 112 - Prevalence of Overweight, Obesity, and Severe Obesity Among Children and Adolescents Ages 2-19, US, 1963-2023
• CDC - Childhood Obesity Facts
• NHS Digital - National Child Measurement Programme, England 2023/24
• WHO - Child Growth Standards
• American Academy of Pediatrics - Childhood Obesity Guidance